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Ptolemy

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   A medieval artist's rendition of Claudius Ptolemaeus
   Enlarge
   A medieval artist's rendition of Claudius Ptolemaeus

   Claudius Ptolemaeus ( Greek: Κλαύδιος Πτολεμαῖος; c. 90 – c. 168 AD),
   known in English as Ptolemy, was a Greek mathematician, geographer,
   astronomer, and astrologer who lived in the Hellenistic culture of
   Roman Egypt. Although no description of his family background or
   physical appearance exists, it is likely he was born in Egypt, probably
   in or near Alexandria.

   Ptolemy was the author of several scientific treatises, three of which
   would be of continuing importance to later Islamic and European
   science. The first is the astronomical treatise that is now known as
   the Almagest (in Greek Η μεγάλη Σύνταξις , "The Great Treatise"). The
   second is the Geography, which is a thorough discussion of the
   geographic knowledge of the Greco-Roman world. The third is the
   astrological treatise known as the Tetrabiblos ("Four books") in which
   he attempted to adapt horoscopic astrology to the Aristotelian natural
   philosophy of his day.

Name

   Claudius is a Roman name. Claudius Ptolemy was almost certainly a Roman
   citizen, and he or his ancestor adopted the nomen of a Roman called
   Claudius, who was in some sense responsible for the citizenship. If, as
   was not uncommon, this Roman was the Emperor, the citizenship would
   have been granted between 14 and 68 CE. The astronomer would also have
   had a praenomen, which we do not know.

   Ptolemy is a Greek name. It occurs once in Greek mythology, and is of
   Homeric form. It was quite common among the Macedonian upper class at
   the time of Alexander the Great, and there are several among
   Alexander's army, one of whom made himself King of Egypt; all the Kings
   after him, until Rome conquered Egypt, were also Ptolemies. Whether the
   astronomer's name represents royal blood, loyalty, or chance is also
   not known.

   Ptolemy read, and wrote in, Greek; he used Babylonian data, probably in
   translation. He worked in Alexandria, which was a primarily Greek and
   Jewish city on the edge of Egypt; there is relatively little evidence
   of native Egyptian inhabitants.

Astronomy

   In the Almagest, one of the most influential books of classical
   antiquity, Ptolemy compiled and extended the astronomical knowledge and
   theories of the ancient Greek and Babylonian world; he relied mainly on
   the work of Hipparchus of three centuries earlier. It was preserved,
   like most of Classical Greek science, in Arabic manuscripts (hence its
   familiar name) and only made available in Latin translation (by Gerard
   of Cremona) in the 12th century. Ptolemy formulated a geocentric model
   that was widely accepted until it was superseded by the heliocentric
   solar system of Copernicus. Likewise his computational methods
   (supplemented in the 12th century with the Arabic computational Tables
   of Toledo) were of sufficient accuracy to satisfy the needs of
   astronomers, astrologers and navigators, until the time of the great
   explorations. They were also adopted in the Arab world and in India.
   The Almagest also contains a star catalogue, which is probably an
   updated version of a catalogue created by Hipparchus. Its list of
   forty-eight constellations is ancestral to the modern system of
   constellations, but unlike the modern system they did not cover the
   whole sky (only the sky Ptolemy could see). The Almagest is also known
   as the Great Syntaxis of Astronomy.

   Ptolemy's astronomy has been wideley criticized for its incorrect
   geocentric view. But according to Manly Hall, an expert on arcane
   wisdom traditions and occult mathematics and science, this was meant to
   describe man's spiritual nature, not the actual astronomical scheme of
   the universe, he wrote "in ridiculing the geocentric system of
   astronomy expounded by Claudius Ptolemy, modern astronomers have
   overlooked the philosophic key to the Ptolemaic system. The universe of
   Ptolemy is a diagram of the relationships existing between the various
   divine and elemental parts of every creature, and is not concerned with
   astronomy as that science is now comprehended. In the above figure,
   special attention is called to the three circles of zodiacs surrounding
   the orbits of the planets. These zodiacs represent the threefold
   spiritual constitution of the universe. The orbits of the planets are
   the Governors of the World and the four elemental spheres in the centre
   represent the physical constitution of both man and the universe.
   Ptolemy's scheme of the universe is a cross section of the universal
   aura, the planets and elements to which he refers having no relation to
   those recognized by modern astronomers." (source:
   http://prs.org/gallery-classic.htm).

   In the Phaseis (Risings of the Fixed Stars) Ptolemy gave a parapegma, a
   star calendar or almanac based on the appearances and disappearances of
   stars over the course of the solar year.

Geographia

   Ptolemy's other main work is his Geographia. This too is a compilation
   of what was known about the world's geography in the Roman Empire
   during his time. He relied mainly on the work of an earlier geographer,
   Marinos of Tyre, and on gazetteers of the Roman and ancient Persian
   Empire, but most of his sources beyond the perimeter of the Empire were
   unreliable.

   The first part of the Geographia is a discussion of the data and of the
   methods he used. Like with the model of the solar system in the
   Almagest, Ptolemy put all this information into a grand scheme. He
   assigned coordinates to all the places and geographic features he knew,
   in a grid that spanned the globe. Latitude was measured from the
   equator, as it is today, but Ptolemy preferred to express it as the
   length of the longest day rather than degrees of arc (the length of the
   midsummer day increases from 12h to 24h as you go from the equator to
   the polar circle). He put the meridian of 0 longitude at the most
   western land he knew, the Canary Islands.
   A 15th century depiction of the Ptolemy world map, reconstituted from
   Ptolemy's Geographia (circa 150), indicating the countries of "Serica"
   and "Sinae" (China) at the extreme right, beyond the island of
   "Taprobane" (Sri Lanka, oversized) and the "Aurea Chersonesus" (Malay
   Peninsula).
   Enlarge
   A 15th century depiction of the Ptolemy world map, reconstituted from
   Ptolemy's Geographia (circa 150), indicating the countries of " Serica"
   and "Sinae" (China) at the extreme right, beyond the island of
   "Taprobane" (Sri Lanka, oversized) and the "Aurea Chersonesus" ( Malay
   Peninsula).

   Ptolemy also devised and provided instructions on how to create maps
   both of the whole inhabited world (oikoumenè) and of the Roman
   provinces. In the second part of the Geographia he provided the
   necessary topographic lists, and captions for the maps. His oikoumenè
   spanned 180 degrees of longitude from the Canary islands in the
   Atlantic Ocean to the middle of China, and about 80 degrees of latitude
   from the Arctic to the East Indies and deep into Africa; Ptolemy was
   well aware that he knew about only a quarter of the globe, and he knew
   that his information did not extend to the Eastern Sea.

   The maps in surviving manuscripts of Ptolemy's Geographia, however,
   date only from about 1300, after the text was rediscovered by Maximus
   Planudes. It seems likely that the topographical tables in books 2-7
   are cumulative texts - texts which were altered and added to as new
   knowledge became available in the centuries after Ptolemy (Bagrow
   1945). This means that information contained in different parts of the
   Geography is likely to be of different date.

   Maps based on scientific principles had been made since the time of
   Eratosthenes ( 3rd century BC), but Ptolemy improved projections. It is
   known that a world map based on the Geographia was on display in Autun,
   France in late Roman times. In the 15th century Ptolemy's Geographia
   began to be printed with engraved maps; the earliest printed edition
   with engraved maps was produced in Bologna in 1477, followed quickly by
   a Roman edition in 1478 (Campbell, 1987). An edition printed at Ulm in
   1482, including woodcut maps, was the first one printed north of the
   Alps. The maps look distorted as compared to modern maps, because
   Ptolemy's data were inaccurate. One reason is that Ptolemy estimated
   the size of the Earth as too small: while Eratosthenes found 700 stadia
   for a degree on the globe, in the Geographia Ptolemy uses 500 stadia.
   It is not certain if these geographers used the same stadion. If they
   both used the traditional Attic stadion of about 185 meters, then the
   older estimate is 1/6 too large, and Ptolemy's value is 1/6 too small.
   See also Ancient Greek units of measurement and History of geodesy.

   Because Ptolemy derived most of his topographic coordinates by
   converting measured distances to angles, his maps get distorted. So his
   values for the latitude were in error by up to 2 degrees. For longitude
   this was even worse, because there was no reliable method to determine
   geographic longitude; Ptolemy was well aware of this. It remained a
   problem in geography until the invention of chronometers at the end of
   the 18th century. It must be added that his original topographic list
   cannot be reconstructed: the long tables with numbers were transmitted
   to posterity through copies containing many scribal errors, and people
   have always been adding or improving the topographic data: this is a
   testimony to the persistent popularity of this influential work in the
   history of cartography.

Astrology

   The mathematician Claudius Ptolemy 'the Alexandrian' as imagined by a
   16th century artist.
   Enlarge
   The mathematician Claudius Ptolemy 'the Alexandrian' as imagined by a
   16th century artist.

   Ptolemy's treatise on astrology, the Tetrabiblos, was the most popular
   astrological work of antiquity and also enjoyed great influence in the
   Islamic world and the medieval Latin West. The Tetrabiblos is an
   extensive and continually reprinted treatise on the ancient principles
   of Horoscopic astrology in four books (Greek tetra means "four", biblos
   is "book"). That it did not quite attain the unrivalled status of the
   Syntaxis was perhaps because it did not cover some popular areas of the
   subject, particularly electional astrology (interpreting astrological
   charts for a particular moment to determine the outcome of a course of
   action to be initiated at that time), and medical astrology.

   The great popularity that the Tetrabiblos did possess might be
   attributed to its nature as an exposition of the art of astrology and
   as a compendium of astrological lore, rather than as a manual. It
   speaks in general terms, avoiding illustrations and details of
   practice. Ptolemy was concerned to defend astrology by defining its
   limits, compiling astronomical data that he believed was reliable and
   dismissing practices (such as considering the numerological
   significance of names) that he believed to be without sound basis.

   Much of the content of the Tetrabiblos may well have been collected
   from earlier sources; Ptolemy's achievement was to order his material
   in a systematic way, showing how the subject could, in his view, be
   rationalized. It is, indeed, presented as the second part of the study
   of astronomy of which the Syntaxis was the first, concerned with the
   influences of the celestial bodies in the sublunar sphere. Thus
   explanations of a sort are provided for the astrological effects of the
   planets, based upon their combined effects of heating, cooling,
   moistening, and drying.

   Ptolemy's astrological outlook was quite practical: he thought that
   astrology was like medicine, that is conjectural, because of the many
   variable factors to be taken into account: the race, country, and
   upbringing of a person affects an individual's personality as much if
   not more than the positions of the Sun, Moon, and planets at the
   precise moment of their birth, so Ptolemy saw astrology as something to
   be used in life but in no way relied on entirely.

Music

   Ptolemy also wrote an influential work, Harmonics, on music theory and
   the mathematics of music. After criticizing the approaches of his
   predecessors, Ptolemy argued for basing musical intervals on
   mathematical ratios (in contrast to the followers of Aristoxenus and in
   agreement with the followers of Pythagoras) backed up by empirical
   observation (in contrast to the overly-theoretical approach of the
   Pythagoreans). Ptolemy wrote about how musical notes could be
   translated into mathematical equations and vice versa in Harmonics.
   This is called Pythagorean tuning because it was first discovered by
   Pythagoras. However, Pythagoras believed that the mathematics of music
   should be based on the specific ratio of 3:2 whereas Ptolemy merely
   believed that it should just generally involve tetrachords and octaves.
   He presented his own divisions of the tetrachord and the octave, which
   he derived with the help of a monochord. Ptolemy's astronomical
   interests also appeared in a discussion of the music of the spheres.

Other works

   In his Optics, a work which survives only in a poor Arabic translation,
   he writes about properties of light, including reflection, refraction
   and colour. The work is a significant part of the early history of
   optics. His other works include Planetary Hypothesis, Planisphaerium
   and Analemma.

Named after Ptolemy

     * Ptolemaeus crater on the Moon.
     * Ptolemaeus crater on Mars.
     * The asteroid 4001 Ptolemaeus.
     * A character in the fantasy series The Bartimaeus Trilogy.

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